1. Field of the Invention
The present invention relates generally to an exhaust gas purification/discharge system for an internal combustion engine. More particularly, the invention is concerned with an apparatus for controlling introduction or feeding of fresh air to an exhaust pipe of an internal combustion engine for promoting purification of engine exhaust gas by catalytic reaction.
2. Description of the Related Art
It is known to introduce or feed fresh air to an exhaust pipe of an internal combustion engine (hereinafter also referred to as the engine) for the purpose of promoting purification of engine exhaust gas through-catalytic reaction. For a better understanding of the present invention, the related art will first be discussed in some detail by reference to FIGS. 8 to 13.
FIG. 8 shows schematically an internal combustion engine system known heretofore. As can be seen in this figure, the engine system is comprised of an internal combustion engine 1, a transmission 2 operatively connected to an output shaft (not shown) of the engine 1, an intake manifold 3 for charging air into the engine 1, two exhaust pipes 4 and 5 for discharging engine exhaust gas, catalytic converters 6 and 7 disposed in the exhaust pipes 4 and 5, respectively, for purifying the exhaust gas through catalytic reaction of carbon monoxide and hydrocarbon contained in the exhaust gas with oxygen contain in the air as introduced, an air pump 8 for taking in and supplying fresh air to the exhaust gas purification/discharge system, a branch pipe 9 for introducing the air taken in through the air pump 8 into the two exhaust pipes 4 and 5 by way of air feed pipes 10 and 11 having inlet ports communicating with the outlet exit ports of the branch pipe 9, and a pair of check valves 12 and 13 installed in the air feed pipes 10 and 11, respectively, for preventing the backward flow of the exhaust gas.
In operation, when the engine is started, the air pump 8. operates to feed air constantly to the exhaust pipes 4 and 5 by way of the pipes 9, 10 and 11 at a substantially uniform flow rate, as shown in FIG. 9. The air introduced into the exhaust pipes 4 and 5 flows through the catalytic converters 6 and 7, during which oxygen contained in the air reacts with carbon monoxide and hydrocarbon contained in the exhaust gas to thereby purify the latter.
The air feeding system of the engine shown in FIG. 8 however suffers from a problem that the quantity of flow air introduced into the exhaust pipe can not be controlled to be optimal for the gas purifying operation of the catalytic converters 6 and 7 because the fresh air is supplied constantly at a substantially uniform flow rate.
FIG. 10 shows schematically a structure of another known engine system equipped with a single exhaust gas purification/discharge system. Referring to the figure, an internal combustion engine 1 is provided with an air intake pipe 3 for charging air into the engine 1, the intake pipe 3 having an inlet port connected to an air cleaner 15 which serves for screening out the dusts or other foreign materials from the air introduced into the intake pipe 3. Mounted within the intake pipe 3 is a throttle valve 14 for adjusting the flow of air charged into the engine 1. An exhaust pipe 4 connected to the engine 1 serves to transport the exhaust gas discharged from the engine 1 to a catalytic converter 6 which is provided for purifying the exhaust gas through the catalytic chemical reaction, as mentioned previously. The intake pipe 3 is connected to the exhaust pipe 4 at a location upstream of the catalytic converter 6 by way of a shunt pipe 9 and an air feed pipe 10 for introducing the air into the exhaust pipe 4 from the intake pipe 3. An air pump 8 is installed in the shunt pipe 9 for feeding the air having passed through the air cleaner 15 into the shunt pipe 9 under pressure. Disposed in the fresh air feed pipe 10 at a position upstream of the exhaust pipe 4 is a check valve 12 for preventing the exhaust gas from flowing backward direction from the exhaust pipe 4 to the intake pipe 3. A transmission 2 is coupled to an output shaft of the engine 1.
In the engine system shown in FIG. 10, the air pump 8 supplies the fresh air at a constant flow rate to the exhaust pipe 4 and hence to the catalytic converter 6, as shown in FIG. 9.
The air feeding system shown in FIG. 10 is disadvantageous in that the quantity of air flow introduced into the exhaust pipe 4 decreases when the exhaust gas pressure within the pipe 4 increases, as a result of which purification efficiency of the catalytic converter 6 is degraded.
In the case of an engine system shown in FIG. 11 which is a modification of that shown in FIG. 10, two catalytic converters, i.e., an auxiliary catalytic converter 6A and a main catalytic converter 6B, are serially installed in the exhaust pipe 4 in this order as viewed in the flow direction of the exhaust gas, wherein a shunt pipe 9 is branched into a first feed pipe 10A connected to the exhaust pipe 4 at a location upstream of the auxiliary catalytic converter 6A and a second feed pipe 10B connected to the exhaust pipe 4 at a location upstream of the main catalytic converter 6B. In this conjunction, it is noted that the auxiliary catalytic converter 6A is usually of a smaller capacity than that of the main catalytic converter 6B so that the former can be activated earlier than the latter.
In operation, the air pump 8 operates in response to the start of the engine to supply the fresh air at a constant flow rate to the exhaust gas discharge system through the shunt pipe 9, as shown in FIG. 9. A part of the air thus introduced flows through the air feed pipe 10A and hence the auxiliary catalytic converter 6A with the other part of air flowing through the air feed pipe 10B and the main catalytic converter 6B for purification of the engine exhaust gas through the catalytic reaction, as mentioned previously. In this engine system, the fresh air is fed to the exhaust gas purification/discharge system at a constant flow rate, as shown in FIG. 9.
The air introduction system of the engine shown in FIG. 11 suffers a problem that the temperature of the exhaust gas within the exhaust pipe 4 as well as that of the catalytic converters 6A and 6B is lowered due to admixture of the fresh air, resulting in that the temperature required for the catalytic reaction is lowered, whereby the exhaust purification efficiency is degraded.
FIG. 12 shows another modification of the engine system shown in FIG. 10. In the case of the engine system shown in FIG. 12, a control valve 16 is installed between the shunt pipe 9 and the air feed pipe 10 at a location upstream of the check valve 12. Further, an electric heater 17 is disposed in the air feed pipe 10 at a location downstream of the check valve 12. Operation of the control valve 16 and that of the electric heater 17 are controlled by a control unit 18.
Upon starting of the engine 1, the air pump 8 operates to feed air from the intake pipe 3 to the exhaust pipe 4 through the shunt pipe 9, the control valve 16, the check valve 12, the electric heater 17 and the air feed pipe 10. The control valve 16 is actuated simultaneously with the start of engine operation to supply the air to the exhaust gas purification/discharge system at a constant flow rate, as shown in FIG. 13. The air thus introduced into the air feed pipe 10 is heated by the electrical heater 17 an then undergoes reaction with carbon monoxide and hydrocarbon contained in the exhaust gas in the course of flowing through the catalytic converter 6.
In the case of the air introduction system shown in FIG. 12, the problem of the system shown in FIG. 11 can certainly be coped with by providing the heater 17 for heating the air to be fed to the exhaust pipe 4. However, temperature rise of the heater 17 which is electrically energized simultaneously with the start of engine operation is usually accompanied with a considerable time lag. As a result of this, a lot of time is taken for heating the air as introduced to a temperature required for the catalytic reaction, giving rise to a problem that the exhaust gas can not satisfactorily be purified in the initial phase of the engine operation.